1. Field of the Invention
The present invention relates generally to a method of fabricating an electrochemical device such as an electric double layer capacitor or a battery, and more particularly, to a method of fabricating a flat type electrochemical device comprising a case having a flat shape.
2. Description of the Prior Art
In an IC card and a thin-type portable electronic apparatus, a thin, that is, a flat power supply device is used. For the power supply device to be constructed in a flat shape, an electric double layer capacitor and a lithium secondary battery are particularly suitable. However, the electric double layer capacitor and the lithium secondary battery use a nonaqueous electrolytic solution. If the moisture content in the inner portion of the power supply device is increased by the entrance of moisture from the exterior, degradation of characteristics such as a decrease in withstand voltage is liable to occur. Consequently, a case structure capable of maintaining a highly sealed state for a long period of time is required.
A flat type electrochemical device having a structure meeting the above described requirements is disclosed in Japanese Patent Laid-Open Gazette No. 140553/1989. The structure disclosed in the prior art is shown in a cross sectional view and a plan view of in FIGS. 2 and 3.
In a flat type electrochemical device 1, a case half body 2a and a case half body 2b made of a thin metal plate or metal foil having a thickness of, for example, not more than 50 .mu.m, are welded in their peripheral portions, thereby constructing a case 2. More specifically, a space is formed in a region inside the peripheral portions of the case half bodies 2a and 2b. A separator 3 and first and second functional substances 4a and 4b for the electrochemical device are contained in the space, and arranged so as to interpose the separator 3 between the functional substances 4a and 4b.
Reference numeral 5 denotes a terminal plate, which is arranged on the first functional substance 4a so as to be electrically connected to the first functional substance 4a. The upper surface of the terminal plate 5 is bonded to an inner flat surface, which faces the containing space, of the first case half body 2a while being separated therefrom by an insulating layer 6.
The electrochemical device 1 has an opening 7 formed in the first case half body 2a, and the terminal plate 5 is exposed at the opening 7, so that the terminal plate 5 and, therefore, the first functional substance 4a can be electrically connected to the exterior.
In the electrochemical device 1, the peripheral portions of the case half bodies 2a and 2b are welded, and the lower periphery of the opening 7 is sealed with the insulating layer 6, thereby sealing the space in the case 2. Moreover, the opening for electrically connecting the functional substance 4a to the exterior is limited to the portion of the insulating layer 6 that faces the opening 7, so that the region from which moisture can enter is relatively small, thereby enhancing sealing performance.
However, the electrochemical device 1 as shown in FIGS. 2 and 3 has the following disadvantages.
The electrochemical device 1 has a disadvantage in that the members contained in the case 2, for example, the separator 3, the functional substances 4a and 4b. Moreover, the like do not have sufficient heat resistance, and the case 2 is easily deformed by distortion because it has an overall flat shape. Accordingly, the welding process that can be used in joining the case half bodies 2a and 2b is limited.